Nearly all conventional displays are active in nature. This means that power must continually be supplied to the displays for them to maintain the images they are displaying. Such conventional displays include direct view and projection cathode-ray tube (CRT) displays, direct view and projection liquid crystal displays (LCD's), direct view plasma displays, projection digital light processing (DLP) displays, and direct view electroluminescent (EL) displays, among others.
Since power must continually be supplied to these types of displays, they can be a significant cause of power usage in devices where supplied power is at a premium, such as portable devices like laptop and notebook computers, personal digital assistant (PDA) devices, wireless phones, as well as other types of portable devices. As a result, designers of such devices usually choose to increase the size of the battery size contained in such devices, increasing weight and cost, or choose to reduce the running time of the devices between battery charges.